High-differential-pressure water electrolysis cell and method of operation

ABSTRACT

An electrolysis cell includes an anode, a cathode and a high-differential-pressure water electrolysis bilayer membrane disposed between the anode and the cathode. The high-differential-pressure bilayer membrane includes a platinum-impregnated ion-exchange membrane layer and an untreated ion-exchange membrane layer. The untreated ion-exchange membrane layer is disposed between the anode and the platinum-impregnated ion-exchange membrane layer.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This U.S. application claims priority from U.S. Provisional ApplicationNo. 61/304,694, filed Feb. 15, 2010, the disclosure of which isincorporated by reference in its entirety.

BACKGROUND

An electrolysis cell is an electro-mechanical assembly that dissociateswater to produce hydrogen and oxygen gases. An electrolysis celltypically includes an anode, a cathode and an ion-exchange membrane(also known as a proton-exchange membrane (PEM)). The ion-exchangemembrane is positioned between the anode and the cathode. Duringoperation, water is electrolyzed to oxygen gas, hydrogen ions (protons)and electrons at the anode. The hydrogen ions migrate from the anode tothe cathode due to an electric field imposed across the ion-exchangemembrane, and the electrons are transferred to the cathode through anexternal circuit by a direct current (DC) power supply. At the cathode,the electrons and the hydrogen ions combine to produce hydrogen gas.

The electrolysis cell consumes water at the anode, and water must becontinuously supplied to the anode for continued operation. In one celldesign, water is fed directly to the anode. In another cell design,water is fed to the cathode and is transported through the ion-exchangemembrane to the anode.

The rate of oxygen generation of an electrolysis cell is governed byFaraday's law in that an increase in cell current provides acorresponding increase in the gas generation rate and consumption ofwater. To meet production requirements, a cell stack can be formed froma number of electrolysis cells or membrane electrode assemblies (MEAs).

SUMMARY

An electrolysis cell includes an anode, a cathode and ahigh-differential-pressure water electrolysis bilayer membrane disposedbetween the anode and the cathode. The high-differential-pressure waterelectrolysis bilayer membrane includes a platinum-impregnatedion-exchange membrane layer and an untreated ion-exchange membranelayer. The untreated ion-exchange membrane layer is disposed between theanode and the platinum-impregnated ion-exchange membrane layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electrolysis cell having a bilayermembrane.

DETAILED DESCRIPTION

High-differential-pressure oxygen generators employing ion-exchangemembrane water electrolyzer technology are being considered forrecharging the primary oxygen system in next generation space suits.Such high-differential-pressure oxygen generators supplied only withwater and electrical power convert water to oxygen on the anode andhydrogen on the cathode. Application requirements dictate that theoxygen is generated at high pressure and hydrogen at low pressure. Forexample, it is desirable to generate oxygen at pressures up to about24,821 kilopascals (3,600 pounds per square inch (psi)) from water atambient pressure and without an external compressor.

FIG. 1 is a schematic diagram of an electrolysis cell 10 which includesanode 12 (having anode electrode 14), cathode 16 (having cathodeelectrode 18), bilayer membrane 20 (having untreated membrane layer 22and platinum-exchanged membrane layer 24), direct current (DC) powersource 26, external circuit 28, water feed line 30, low-pressurehydrogen 32 and high-pressure oxygen 34. Anode 12 is located on one sideof electrolysis cell 10, and cathode 16 is located opposite of anode 12.Bilayer membrane 20 is positioned between anode 12 and cathode 16 suchthat untreated membrane layer 22 is adjacent anode 12 andplatinum-exchanged membrane layer 24 is adjacent cathode 16. DC powersource 26 is connected to anode electrode 14 and cathode electrode 18 byexternal circuit 28 and supplies electrical power to electrolysis cell10. Water feed line 30 supplies liquid water to cathode 16.

During operation, water is introduced into anode 12 where it contacts ananode catalyst and is electrolytically dissociated into hydrogen ions(protons), electrons and oxygen gas according to equation (1).

H₂O→½O₂+2H⁺+2e ⁻  (1)

The hydrogen ions migrate from anode 12 through bilayer membrane 20 tocathode 16 under the effect of the electric field imposed acrosselectrolysis cell 10 by DC power source 26. The electrons aretransferred through external circuit 28 by DC power source 26. A cathodecatalyst of cathode 16 recombines the hydrogen ions and the electrons toproduce low-pressure hydrogen 32 according to equation (2).

2H⁺+2e ⁻→H₂  (2)

Anode electrode 14 and cathode electrode 18 are hydrophilic. For theelectrolysis reactions to occur cathode electrode 18 takes water from abulk stream and anode electrode 14 absorbs water from membrane 20. Anodeelectrode 14 is also known as the oxygen electrode because high-pressureoxygen 34 is produced at anode 12, and cathode electrode 18 is alsoknown as the hydrogen electrode because low-pressure hydrogen 32 isproduced at cathode 16.

Electrolysis cell 10 is a high-differential-pressure electrolysis cellwhich produces high-pressure oxygen 34 at anode 12 and low-pressurehydrogen 32 at cathode 16. In one example, oxygen can be produced atpressures of up to about 24,821 kilopascals (3,600 pounds per squareinch (psi)) from water at ambient pressure. Low-pressure hydrogen 32 isproduced at about the same pressure as the water and is typicallyslightly above ambient pressure in order to overcome the pressure dropof flowing water through cathode 16. In electrolysis cell 10, anode 12is at a higher pressure than cathode 16, and a pressure differentialexists across bilayer membrane 20.

As shown in FIG. 1, water is provided at cathode 16 by water feed line30. The water moves from cathode 16 through bilayer membrane 20 to anode12 where it is consumed. As described further below, providing water tocathode 16 reduces the number of components, the overall weight andenhances the reliability of an oxygen generation system employingelectrolysis cell 10.

Bilayer membrane 20 improves water transport in electrolysis cell 10 andincreases the oxygen generation rate for a given cell stack size.Bilayer membrane 20 is a high-differential-pressure membrane. A supportstructure enables operation of bilayer membrane 20 at high-differentialpressure.

Bilayer membrane 20 includes untreated membrane layer 22 andplatinum-exchanged membrane layer 24. Untreated membrane layer 22 andplatinum-exchanged membrane layer 24 are in continuous contact with oneanother. Continuous contact between membrane layers 22 and 24 isimportant for water transport through bilayer membrane 20 and results inreduced contact resistance. Untreated membrane layer 22 andplatinum-exchanged membrane layer 24 can be stacked adjacent to oneanother as two separate structures. Alternatively, untreated membranelayer 22 and platinum-exchanged membrane layer 24 can be hot-pressedtogether to form a single structure. Hot-pressing membrane layers 22 and24 together reduce interfacial resistance between membrane layers 22 and24 when membrane layers 22 and 24 are dry. Experimental testing hasshown that there is no resistance penalty for hot-pressing ion-exchangemembrane layers together. In one example, untreated membrane layer 22and platinum-exchanged membrane layer 24 are hot-pressed together atabout 1723 kilopascals (250 psi) and 130 degrees Celsius.

In one example, untreated membrane layer 22 is thicker thanplatinum-exchanged membrane layer 24. Bilayer membrane 20 has thicknessT_(T), untreated membrane layer 22 has thickness T_(U) andplatinum-exchanged membrane layer 24 has thickness T_(P). In oneexample, thickness T_(u) of untreated membrane layer 22 is about 0.18 mm(0.007 inches) and thickness T_(P) of platinum-exchanged membrane layer24 is about 0.05 mm (0.002 inches). In another example, the ratio ofT_(P) to T_(T) is between about 0.002 and about 0.9999. In a furtherexample, the ratio of T_(P) to T_(T) is between about 0.002 and about0.8. In a still further example, the ratio of T_(P) to T_(T) is betweenabout 0.004 and about 0.5.

Untreated membrane layer 22 is located closer to anode 12 than tocathode 16. Untreated membrane layer 22 can be an ion-exchange membrane,such as a Nafion® membrane, a perfluorocarbon ion-exchange membraneproduced by E.I. du Pont de Nemours & Co. of Wilmington, Del. Membranelayer 22 is referred to as an “untreated membrane layer” becausemembrane layer 22 has not been treated with platinum particles. However,membrane layer 22 can be subjected to treatments other than impregnationof platinum particles. For example, untreated membrane layer 22 can be achemically-stabilized membrane, such as a chemically-stabilized Nafion®membrane. A chemically-stabilized membrane does not contain or containsonly a small number of unterminated polymer chains and is lesssusceptible to free-radical attack.

Platinum-exchanged membrane layer 24 is located between untreatedmembrane layer 22 and cathode 16. Platinum-exchanged membrane layer 24can be an ion-exchange membrane, such as a perfluorocarbon ion-exchangemembrane, impregnated with a platinum catalyst. For example,platinum-exchanged membrane layer 24 can be a platinum-impregnatedion-exchange membrane. An example method of impregnating an ion-exchangemembrane with platinum is provided in U.S. Pat. No. 5,342,494 by Shaneet al., which is herein incorporated by reference. The method includesconditioning the membrane by preferentially exchanging the hydrogen ionof the membrane's acid group with replacement cations which are largerthan the hydrogen ions to condition the membrane, contacting theconditioned membrane with solution containing platinum ions such thatthe platinum ions exchange with the replacement cations, and convertingthe impregnated platinum ions to their metal form to form a platinumcatalyst. The amount of platinum impregnated in the membrane depends onthe type of membrane to be impregnated. For example about 0.92 grams ofplatinum per square meter per mil (gm Pt/m²/mil) (0.086 gm Pt/ft²/mil)can be impregnated into a Nafion® membrane by a single impregnationprocedure. Multiple impregnation procedures may be necessary to attainthe platinum loading required to produce the desired oxygen purity. Inone example, platinum-exchanged membrane layer 24 has a platinum loadingof about 4.6 to about 36 mg/cm³ of membrane material (about 75 to about590 mg/in³ of membrane material).

The platinum of platinum-exchanged membrane layer 24 is dispersed orimpregnated throughout membrane layer 24. The platinum ofplatinum-exchanged membrane layer 24 is configured for catalyticactivity. The platinum forms catalyst sites in the platinum-impregnatedion-exchange membrane layer. As described below, the platinumfacilitates hydration of bilayer membrane 20 and improves watertransport through bilayer membrane 20.

In one example, platinum is impregnated on platinum-exchanged membranelayer 24 before forming bilayer membrane 20. Forming bilayer membrane 20from separate membrane layers 22 and 24 simplifies production becauseplatinum is impregnated on the entire platinum-exchanged membrane layer24 and not only on select regions.

The rate of oxygen generation of electrolysis cell 10 is governed byFaraday's law in that an increase in cell current provides acorresponding increase in the gas generation rate and consumption ofwater. Therefore, increasing the oxygen generation rate increases theconsumption of water at anode 12, and in order to maintain an increasedoxygen generation rate for a given stack size, anode 12 must be suppliedwith sufficient water.

In electrolysis cell 10, water is fed to cathode 16 by water feed line30 and is then transported through bilayer membrane 20 to anode 12. Morespecifically, the water is transported through platinum-exchangedmembrane layer 24 and then untreated membrane layer 22 before reachinganode 12. Feeding water to high-pressure anode 12 necessitatesdeveloping a high-pressure oxygen/water phase separator, incorporating ahigh-pressure water pump in the anode loop and ahigh-differential-pressure feed pump. These elements add weight, volumeand complexity to a complete water electrolysis system. In contrast,feeding water to low-pressure cathode 16 as shown in FIG. 1, reduces thenumber of components and the overall weight of and enhances thereliability of a water electrolysis system employing electrolysis cell10.

Continuous operation of electrolysis cell 10 requires water transportfrom cathode 16 to anode 12 where it is consumed. Several mechanisms inaddition to consumption deplete anode 12 of water and inhibit the anodereaction. First, hydrodynamic pressure across membrane 20 depletes anode12 of water. Water is supplied to anode 12 against the hydrodynamicpressure differential across electrolysis cell 10. The pressure of anode12 is higher than that of cathode 16, such that the hydrodynamicpressure “pushes” against water moving to anode 12.

Electro-osmotic drag also depletes anode 12 of water. Electro-osmoticdrag occurs when protons from anode 12 “drag” water molecules from anode12 to cathode 16 as they move through membrane 20. For example, protonmovement from anode 12 to cathode 16 can drag up to six molecules ofwater per proton through a Nafion® ion-exchange membrane.

The water vapor of anode 12 is in equilibrium with the water containedwithin membrane 20. Therefore, as the water vapor leaves anode 12 withthe product oxygen, it is replenished with water from membrane 20.

Water transportation from cathode 16 to anode 12 occurs by two primarymechanisms: diffusion and capillary action. Diffusion occurs because theconcentration of water on cathode 16 is larger than that of anode 12. Inion-exchange membranes, such as Nafion®, the magnitude of the diffusionconstant is a function of the water content of the membrane such thatthe diffusion rate is lower when the membrane has a lower water content.Therefore, once membrane 20 begins to lose water, it becomes harder tosupply water to anode 12.

Capillary action is another water transport mechanism. Capillary actionfollows the law of Young and Laplace where P_(c)=4γ cos(θ)/d, whereP_(c) is the capillary pressure, γ is the surface tension of water, θ isthe contact angle and d is the diameter of the capillary. For Nafion®ion-exchange membranes, capillary action occurs because the pores shrinkwhen they dry. This results in a capillary pressure differential betweencathode 16 and anode 12.

The platinum of platinum-exchanged membrane layer 24 protects againstfree-radical attack. Free-radical attack can create pinholes inion-exchange membranes of water electrolysis cells and can eventuallyresult in cell stack failure. As described further below, the locationof platinum in bilayer membrane 20 can be limited to where free-radicalattack is most likely to occur and where the most benefit is achieved.

The platinum of platinum-exchanged membrane layer 24 also improves thepurity of the oxygen and hydrogen gas produced by electrolysis cell 10.Hydrogen and oxygen diffuse through bilayer membrane 20 due toconcentration gradients. For example, hydrogen can diffuse to anode 12and oxygen can diffuse to cathode 16. This diffusion (also known ascross-diffusion) reduces the hydrogen and oxygen purities. The platinum(or catalyst sites) of platinum-exchanged membrane layer 24 recombinesthe oxygen and hydrogen cross-diffusing through bilayer membrane 20 toform water. This reduces hydrogen contamination of oxygen produced atanode 12 and oxygen contamination of hydrogen produced at cathode 16. Inone example, platinum-exchanged membrane layer 24 provides sufficientsites to produce an oxygen purity of at least 99.5%.

Bilayer membrane 20 improves water transport to anode 12 while producinghigh-purity products in high-differential-pressure water electrolysiscell 10. Anode 12 can oxidize platinum metal. Locatingplatinum-exchanged membrane layer 24 closest to cathode 16 minimizesoxidization of the platinum metal by anode 12.

Platinum also increases the pore diameter of ion-exchange membranes.Ion-exchange membranes such as Nafion® comprise a plurality ofnano-sized pores which transport water from one side of electrolysiscell 10 to the other. Using platinum-exchanged membrane layer 24 alonewould result in a reduced capillary pressure, which would hinder watertransport against the high-differential pressure and reduce hydrogen andoxygen generation. By increasing the pore diameter of only a selectportion (i.e. platinum-exchanged membrane layer 24) of bilayer membrane20, the reduced capillary action is limited. In bilayer membrane 20, thelarger pores of platinum-exchanged membrane layer 24 enhance diffusionof water through layer 24 while the small pores of untreated membranelayer 22 facilitate improved water transport to anode 12 against thepressure gradient. The small pores of untreated membrane layer 22improve the capillary pressure towards anode 12 and increase thelimiting current. Thus, bilayer membrane 20 improves water transport toanode 12 and results in increased oxygen production at the same stackmass and volume.

Additionally, the pore size of membrane layers 22 and 24 affects theoxygen purity. The small pores of untreated membrane layer 22 limit theamount of gas crossover or cross-diffusion in water electrolysis cell10. Increasing the pore size of membrane layers 22 and 24 reduces theresistance to cross-diffusing gases. It is particularly important thatoxygen has resistance to transport when the oxygen pressure is muchgreater than that of hydrogen.

Operation of electrolysis cell 10 can cause degradation to ion-exchangemembranes, for example from free-radical attack. Membrane degradation istypically most severe in a plane parallel and adjacent to cathode 16.The thickness of this plane is approximated by the product of thicknessT_(T) of bilayer membrane 20 and PH₂/(PH₂+PO₂), where PH₂ is the partialpressure of hydrogen and PO₂ is the partial pressure of oxygen. Inhigh-differential-pressure electrolysis cell 10, the partial pressure ofoxygen is much higher than the partial pressure of hydrogen. Thus, avery thin plane adjacent to the cathode will experience the most severedegradation. In electrolysis cell 10, the plane that is predicted toexperience the most severe degradation and free-radical attack can beprotected by the platinum of platinum-exchanged membrane layer 24.Platinum-exchanged membrane layer 24 can be designed to encompass thecalculated plane. The membrane experiences little degradation outside ofthis plane, such that untreated membrane layer 22 does not experiencesignificant degradation despite not being protected by platinum.

During operation of electrolysis cell 10, protons from anode 12 aretransferred through bilayer membrane 20 to cathode 16. The protons aretransferred through bilayer membrane 20 in their hydrated form. Thisreduces the water transfer rate through bilayer membrane 20 and resultsin limited current density due to the limited water availability atanode 12. Bilayer membrane 20 increases water transport in electrolysiscell 10, resulting in a larger limited current density and increasedoxygen generation for a given stack size.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. An electrolysis cell comprising: an anode; a cathode; and ahigh-differential-pressure water electrolysis bilayer membrane disposedbetween the anode and the cathode, the high-differential-pressure waterelectrolysis bilayer membrane comprising: a platinum-impregnatedion-exchange membrane layer; and an untreated ion-exchange membranelayer disposed between the anode and the platinum-impregnatedion-exchange membrane layer.
 2. The electrolysis cell of claim 1,wherein the untreated ion-exchange membrane layer is thicker than theplatinum-impregnated ion-exchange membrane layer.
 3. The electrolysiscell of claim 1, wherein a ratio of a platinum-impregnated ion-exchangemembrane layer thickness to a high-differential-pressure waterelectrolysis bilayer membrane thickness is between about 0.002 to about0.999.
 4. The electrolysis cell of claim 3, wherein the ratio of theplatinum-impregnated ion-exchange membrane layer thickness to thehigh-differential-pressure water electrolysis bilayer membrane thicknessis between about 0.002 and about 0.8
 5. The electrolysis cell of claim3, wherein the ratio of the platinum-impregnated ion-exchange membranelayer thickness to the high-differential-pressure water electrolysisbilayer membrane thickness is between about 0.004 and about 0.5.
 6. Theelectrolysis cell of claim 1, wherein the platinum-impregnatedion-exchange membrane layer and the untreated ion-exchange membranelayer are hot-pressed together to form a single structure.
 7. Theelectrolysis cell of claim 1, wherein the untreated ion-exchangemembrane layer is a chemically-stabilized perfluorocarbon ion-exchangemembrane.
 8. The electrolysis cell of claim 1, wherein the electrolysiscell is a high-differential-pressure water electrolysis cell thatgenerates oxygen at a pressure up to about 24,821 kilopascals.
 9. Theelectrolysis cell of claim 1, and further comprising: a water feed lineconnected to the cathode.
 10. A method for producing high-purity oxygen,the method comprising: feeding water to a cathode of ahigh-differential-pressure electrolysis cell, thehigh-differential-pressure cell comprising an anode and a bilayermembrane disposed between the anode and the cathode, the bilayermembrane including a platinum-impregnated ion-exchange membrane layerand an untreated ion-exchange membrane layer, wherein theplatinum-impregnated ion-exchange membrane layer is impregnated withplatinum which forms catalyst sites in the platinum-impregnatedion-exchange membrane layer; transporting water from the cathode to theanode across the platinum-impregnated ion-exchange membrane layer andthen across the untreated ion-exchange membrane layer; electrolyzing thewater in the anode to produce hydrogen ions, oxygen gas and electrons;passing the electrons through an exterior circuit to the cathode;passing the hydrogen ions through the untreated ion-exchange membranelayer and then through the platinum-impregnated ion-exchange membranelayer to the cathode; combining the hydrogen ions and the electrons toform hydrogen gas in the cathode; and recombining cross-diffusinghydrogen gas of the cathode and oxygen gas of the anode at the catalystsites of the platinum-impregnated ion-exchange membrane.
 11. The methodof claim 10, wherein the anode is configured to operate at pressures upto about 24,821 kilopascals.
 12. The method of claim 11, wherein theplatinum-impregnated ion-exchange membrane layer and the untreatedion-exchange membrane layer are hot-pressed together to form a singlestructure.
 13. The method of claim 12, wherein a ratio of a thickness ofthe platinum-impregnated ion-exchange membrane layer to a thickness ofthe bilayer membrane is between about 0.002 and about 0.999.
 14. Themethod of claim 13, wherein the ratio of the thickness of theplatinum-impregnated ion-exchange membrane layer to the thickness of thebilayer membrane is between about 0.002 and about 0.8.
 15. The method ofclaim 10, wherein the platinum-impregnated ion-exchange membrane layeris adjacent the cathode and the untreated ion-exchange membrane layer isadjacent the anode.
 16. A system for producing high-purity oxygen, thesystem comprising: an oxygen anode for producing hydrogen ions, oxygengas and electrons; a hydrogen cathode for producing hydrogen gas; awater feed connected to the cathode for feeding water thereto; and anelectrolysis bilayer ion-exchange membrane comprising: aplatinum-impregnated ion-exchange membrane layer; and an untreatedion-exchange membrane layer, wherein the bilayer ion-exchange membraneis disposed between the anode and the cathode such that theplatinum-impregnated ion-exchange membrane layer is closer to thecathode than to the anode.
 17. The system of claim 16, wherein a ratioof a thickness of the platinum-impregnated ion-exchange membrane layerto a thickness of the electrolysis bilayer ion-exchange membrane isbetween about 0.002 to about 0.999.
 18. The system of claim 16, whereinthe ratio of the thickness of the platinum-impregnated ion-exchangemembrane layer to the thickness of the electrolysis ion-exchange bilayermembrane is between about 0.004 and about 0.5.
 19. The system of claim16, wherein the platinum-impregnated ion-exchange membrane layer and theuntreated ion-exchange membrane layer are hot-pressed together to form asingle structure.
 20. The system of claim 16, wherein theplatinum-impregnated ion-exchange membrane layer and the untreatedion-exchange membrane layer are separate structures in the bilayerion-exchange membrane.